Congrats on the new ride! Looks like a good choice. Wish my first mountain bike had hydraulic disc instead of cantilever brakes.
Lots of good advice on fit from everybody so far. Since this is your first mountain bike I wouldn’t worry too much about setting your saddle height perfect until you’ve had a chance to ride a short distance on some dirt. You might want to be able to put a foot down pretty quick and that might be tricky with your saddle set to maximize efficiency.
Once you get more comfortable on the trail, then work on getting that saddle height dialed in by raising it a little at a time. 1/4 inch or less increments. Compared to your beach cruiser your saddle will seem sky-high, but you will be able to ride much further and tackle more obstacles with less effort.
A great resource for all things bicycle, including fit issues
https://www.sheldonbrown.com/
The website is a little dated and there’s quite a few ads, but Sheldon was awesome, RIP
Now go ride.